1. Field of the Invention:
The present invention relates to a control channel transmission/reception method and apparatus for use in a wireless communication system. More particularly, the present invention is related to a method for determining a transmission scheme guaranteeing reliability of the information carried by the signals transmitted at a high order diversity level on the mobility channels with dynamic diversity in time and frequency domains.
2. Description of the Related Art:
In a wireless communication system including at least one evolved Node B (eNB) and at least one User Equipment (UE), the eNB schedules Downlink (DL) transmission to the UE and Uplink (UL) transmission from the UE. The UL and DL scheduling is performed in subframe units, and the scheduling information is transferred from the eNB to the UE through the control channel at each DL subframe.
In the following description, the 3rd Generation Partnership Project (3GPP) Long-Term Evolution (LTE) releases 8 to 10 systems are referred to as legacy system(s) while the release 11 or later systems are referred to as proposed system(s) that may be implemented according to exemplary embodiments of the present invention. Exemplary embodiments of the present invention are also capable of being applied to other cellular systems.
The downlink data is transmitted through Physical Downlink Shared Channel (PDSCH). Downlink Control Information (DCI) includes Downlink Channel Status Information (DL CSI) feedback request per UE and Uplink Transmission Scheduling Assignments (UL SAs) for uplink transmissions of the UEs or Downlink Scheduling Assignments (DL SAs) for UEs to receive PDSCHs. The scheduling assignments (SAs) are transmitted through DCI formats carried by Physical Downlink Control Channels (PDCCHs). In addition to the SAs, the PDCCHs carry a DCI common in all UEs or a group of UEs.
The 3GPP LTE-Advanced (LTE-A) system adopts Orthogonal Frequency Division Multiple Access (OFDMA) for downlink in which the system bandwidth is divided into multiple subcarriers. A group of 12 consecutive subcarriers is referred to as Resource Block (RB). An RB is the basic unit of resource allocation in the LTE/LTE-A system.
FIG. 1 is a diagram illustrating the structure of a resource allocation unit for use in an LTE/LTE-A system according to the related art.
Referring to FIG. 1, the basic unit of resource allocation of the LTE/LTE-A system is a subframe in the time domain. As shown in FIG. 1, a subframe consists of 14 consecutive Orthogonal Frequency Division Multiple (OFDM) symbols. Resource Element (RE) is the smallest unit, and is made up of 1 OFDM symbol x one subcarrier. A single modulation symbol is mapped to a resource element.
As shown in FIG. 1, different time and frequency resources can be used for transmitting different types of signals. A Cell-specific Reference Signal (CRS) is transmitted to support UE mobility in such a situation of initial attach and handover and legacy PDSCH transmission modes. The Demodulation Reference Signal (DMRS) is transmitted to support new PDSCH transmission modes. The control channels are transmitted to notify the UE of the size of control region and DL/UL scheduling assignments (SAs) and ACK/NACK for UL HARQ operation.
The Channel Status Information Reference Signal (CSI-RS) is the reference signal for use in downlink channel measurement of the UE for CSI feedback. The CSI-RS may be transmitted in a certain group of resource elements marked with indices A to J. In addition, the zero power CSI-RS or muted CSI-RS can be configured in the case where the resource elements marked with the indices A to J are not used for transmitting any of reference signals, data signals, and/or control signals. The zero power CSI-RS or muted CSI-RS is used for enhancing the measurement performance of the UEs receiving the CSI-RSs from neighbor transmission points in the LTE-A system.
PDSCH is transmitted in the data region corresponding to the REs not used for transmission of any of CRS, DMRS, CSI-RS, and zero power CSI-RS.
As described above, the eNB transmits PDCCH for various purposes such as UL/DL scheduling allocation and CSI-RS feedback request indication in the legacy LTE/LTE-A system. In the original characteristics of the OFDMA system which achieves system throughput enhancement with multi-user transmission and frequency selective scheduling, it becomes necessary to transmit multiple PDCCHs addressed to the multiple UEs. In addition, the adoption of Multi-User Multiple Input Multiple Output (MU-MIMO) for PDSCH transmission to spatially distributed UEs requires simultaneous transmission of PDCCHs to the multiple UEs.
In 3GPP releases 8 to 10, the control channel is transmitted at the beginning of the subframe in order for the UE to acquire the scheduling information quickly and thus perform data decoding efficiently. The Physical Downlink Control Channel (PDCCH) is configured to be transmitted 1 to 3 OFDM symbols at the beginning of the subframe.
The number of the OFDM symbols for use in transmitting PDCCH is indicated by Physical Control Format Indication Channel (PCFICH) mapped to the first OFDM symbol. One PDCCH is carried by L Control Channel Elements (CCEs). L denotes CCE aggregation level and can be 1, 2, 4, or 8. A CCE consists of 36 subcarriers distributed across the system bandwidth.
In order to secure the control channel resource enough for transmitting UL/DL resource allocation information, Enhanced Physical Data Control Channel (E-PDCCH or ePDCCH) has been introduced as a new Control Channel (CCH) to overcome the shortage of PDCCH capacity in LTE-A release 11. In the case of E-PDCCH, a newly defined Enhanced Control Channel Element (E-CCE) replaces the legacy CCE. It differs from the CCE in that E-CCE is defined in the data region rather than the control region of the resource block.
The major cause for the PDCCH resource shortage is that the PDCCH transmission is transmitted in 1 to 3 OFDM symbols at the beginning of the subframe. Furthermore, the frequent MU-MUMO transmission with scheduling multiple UEs on the same frequency and time resources aggravates lack of PDCCH resource significantly. In order to address this issue, E-PDCCH is designed to be transmitted in the data region of a subframe along with PDSCH unlike the legacy PDCCH. The DMRS is the reference signal designed for the E-PDCCH.
A description of PDCCH transmission is described below.
At the transmitter side, the eNB adds user-specific Cyclic Redundancy Checks (CRCs) to multiple PDCCHs, encodes CRC-added PDCCHs independently depending on CCE aggregation level of 1, 2, 4, or 8, performs rate-matching on the encoded PDCCHs, multiplexes the rate-matched PDCCHs, and maps the multiplexing result to PDCCH resources. At the receiver side, the UE estimates the CCE aggregation level and searches for the PDCCHs in a predetermined search space using the user-specific CRC. Before locating and identifying PDCCHs, the UE performs blind decoding of a set of candidate channels to determine which contains its signaling information.
In the 3GPP LTE releases 8 to 10, PDCCH is transmitted using Space Frequency Block Code (SFBC) through multiple transmit antennas of the eNB. SFBC is a transmission scheme in order for the UE to receive the single modulation symbol transmitted by the eNB with diversity order 2. Assuming a channel h1 from the eNB's transmit antenna 1 to the UE and another channel h2 from the eNB's transmit antenna 2 to the UE, the SFBC transmission allows the UE to recover the modulation signal scaled to |h1|2+|h2|2. If the modulation signal is received through |h1|2+|h2|2, this means that the modulation signal has been transmitted with diversity order 2. Without application of the SFBC transmission scheme, only the diversity order 1 is achievable in the flat fading channel environment. Typically, if high diversity order is possible, this means that the transmission signal is robust to the radio channel variation in the time or frequency domain. By achieving high diversity order, it is possible to recover the original signal with low error probability as compared to the case of a low diversity order.
In 3GPP, SFBC is performed with CRC as common reference signal used for multiple UEs attached to the same cell. In addition, the SFBC is transmitted. The SFBC-based transmit diversity scheme is advantageous in the situation of significant channel variation in time or frequency domain. By spreading the transmission signal widely, it is possible to achieve high diversity order and thus secure the throughput reflecting average state of the radio channel.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present invention.